Method for depositing metal oxide films

ABSTRACT

A method for depositing a metal oxide film on a surface of a supporting body for the film, comprising the steps of: —providing a deposition chamber; —providing a pulsed beam of electrons and plasma in the deposition chamber; —supplying a supporting body in the deposition chamber, the supporting body having a deposition surface; —providing a target body made of a material which comprises the metal oxide in the deposition chamber, the target body having a target surface; —forming a plume of metal oxide ablated from the target surface by means of the impact of the pulsed beam of electrons and plasma against the target surface; and —depositing a metal oxide film on the deposition surface by means of the contact of the plume with the deposition surface.

TECHNICAL FIELD

The invention relates to a method for depositing metal oxide films, tometal oxide films which can be obtained with such method, and to deviceswhich contain such films. In particular, the invention relates to amethod for depositing thin films of transparent conducting oxides (TCO)on surfaces of flexible materials and on surfaces of rigid materialswhich are preferably transparent.

BACKGROUND ART

Metal oxide films, particularly thin metal oxide films which combineconduction and transparency properties and zinc oxide, zinc oxide dopedwith aluminum (AZO), lithium oxide (LZO), and other dopants, have beenused extensively as transparent conducting electrodes in optoelectronicdevices such as solar cells and flat panel displays (FPD), surfaceheaters in motor vehicle windows, lenses of cameras and mirrors, andalso as materials for heat-reflecting transparent windows for buildings,lamps and solar collectors. They are also used extensively as an anodecontact in organic light emitting diodes (OLEDs).

Several deposition methods are known and are used to grow these films,particularly TCO, including chemical vapor deposition (CVD), magnetronsputtering (arc- or radio frequency-based), thermal evaporation, andspray pyrolysis. These techniques require a complex process forpreparing and using the initial materials from which the oxides thenform in the environment of a deposition chamber. These techniquesfurther require rather high temperatures of the substrate and/or of thesubsequent thermal treatments and therefore do not allow to use plasticsubstrates, which would be damaged or even melted by high temperatures.

The method of growing by using pulsed lasers has been shown to exceedthis limitation. Moreover, the pulsed laser deposition method (PLD) hasyielded satisfactory results as regards the uniformity of the film andthe chemical purity of the conducting transparent oxides deposited bymeans of this technique. However, the cost of laser sources posesimportant problems in the use of this method on an industrial level asregards the cost of purchasing the apparatus, the cost of production andsystem maintenance, and the effectiveness of the ablation process, sincethe PLD method, which uses photons as energy carriers for ablation, isnot suitable for depositing transparent oxides (poor interaction of thephotons with transparent material), and scalability.

Where flexibility and safety are important, glass cannot be used, sinceit is very fragile and too heavy, especially for large screens. Thesedisadvantages can be overcome by using plastic surfaces or metal sheets,which can be very light and flexible. The development of an advancedOLED technology based on plastic or metal sheet supports requires atransparent conducting oxide material to be grown directly on plasticsor on an organic emitting layer for the geometry of the metallic sheet.Passive and active matrix displays, such as liquid crystal displays(LCDs) and electroluminescent organic displays, will benefit greatlyfrom the use of flexible surfaces.

If it is instead necessary to deposit TCO films over the emittingorganic layer in OLEDs, the sputtering technique cannot be used to growthe electrode film, since energy species at more than 100 eV originatingfrom the sputtering target damage the organic layer.

The method currently applied to deposit transparent conducting oxidefilms on plastic surfaces by sputtering produces a rough surfacemorphology and a high resistivity, which degrades the performance ofOLEDs.

Accordingly, there is a great need for conducting transparent thin filmson flexible surfaces which have a smooth surface, high opticaltransparency and low electrical resistivity and are suitable for use inOLEDs, and for methods for producing such films.

DISCLOSURE OF THE INVENTION

The aim of the present invention is to provide metal oxide films,particularly thin films of transparent conducting oxides (TCO),preferably on flexible surfaces, which have a smooth surface, highoptical transparency and low electrical resistance, and a method forproducing them.

Another object of the present invention is to provide a method whichallows to grow films made of TCO material directly on plastics or overan emitting organic layer, for use in passive and active matrix displayssuch as liquid crystal displays (LCDs) and electroluminescent organicdisplays which benefit greatly from the use of flexible surfaces, andfor use in advanced OLED technology based on plastic supports or metalsheets.

This aim and these and other objects which will become apparent from thedescription that follows are achieved by a method according to thepresent invention for depositing a metal oxide film on a surface of asupporting body for said film, which comprises the steps of:

providing a deposition chamber;

providing a pulsed beam of electrons and plasma in said depositionchamber;

supplying a supporting body in said deposition chamber, said supportingbody having a deposition surface;

providing a target body made of a material which comprises said metaloxide in said deposition chamber, said target body having a targetsurface;

providing a plume—a cloud of plasma (ionized hot gas) of metal oxideablated from said target surface by means of the impact of said pulsedbeam of electrons and plasma against said target surface;

depositing a metal oxide film on said deposition surface by means of thecontact of said plume with said deposition surface.

In a preferred embodiment of the present invention, the metal oxide is atransparent conducting oxide, particularly a metal oxide selected fromthe group constituted by zinc oxide, zinc oxide doped with aluminum,such as a material composed of 90% to 100% by weight of ZnO and 10% to0% by weight of Al.

The support used in the method according to the present invention forthe deposition of the film can be a rigid support or a flexible supportand can be a support made of a solid inorganic material, such as glass,quartz, and ZnSe, CdS, different types of metal and inorganicsemiconductor, et cetera, or it can be a support made of solid organicmaterial, a material selected from the group constituted by polymerssuch as polyesters, polyolefines, polyimides, phenolic resins,polyanhydrides, conducting polymers, conjugated polymers,fluoropolymers, silicone rubbers, silicone polymers, biopolymers,copolymers, block copolymers such as polycarbonate, PTFE, PET, PNT,PEDOT, polyaniline, polypyrrole, polythiophenes, polyparaphenylenes,(PPV), polyfluorenes, and molecular solids like molecularsemiconductors, molecular crystals, molecular thin films, moleculardyes, such as AlQ3, thiophene oligomers, PPV oligomers, pentacene,tetracene, rubrene, NPB, fullerenes, carbon nanotubes and fullerides.

In a particularly preferred embodiment of the present invention, thedeposited metal oxide film is a thin film, even of nanometer-scalethickness, of a transparent conducting oxide (TCO), and the support onwhich the film is deposited is a flexible support (i.e., a support whichcan be rolled up without damaging it).

The flexible supports used in the method according to the presentinvention can be made for example of a solid organic material, such aspolycarbonate, PTFE, PET, AlQ3, T6, T7, PEDOT, PPV, αNPB, et cetera, orcan be a metallic sheet.

The film or thin film of transparent conducting oxide (TCO) can be afilm or thin film of transparent conductive oxide, particularly a metaloxide selected from the group constituted by zinc oxide, zinc oxidedoped with aluminum, such as a material composed of up to 100% to 90% byweight of ZnO and 0% to 10% by weight of Al.

Another aspect of the present invention relates to a metal oxide film,particularly a thin film of transparent conducting oxide, which can beobtained by means of the method according to the present invention.

Another aspect of the present invention relates to a method fordepositing a film of a metal oxide doped with a doping agent on asurface of a supporting body for said film, comprising the steps of:

providing a deposition chamber;

providing a first and a second pulsed beam of electrons and plasma insaid deposition chamber;

supplying a supporting body in said deposition chamber, said supportingbody having a deposition surface;

providing in said deposition chamber a first and a second target body,said first target body being made of a material which comprises saidmetal oxide, said second target body being made of a material whichcomprises said doping agent, said first target body having a firsttarget surface and said second target body having a second targetsurface;

providing a plume of metal oxide ablated from said first target surfaceby means of the impact of said first pulsed beam of electrons and plasmaagainst said first target surface, and a plume of said doping agentablated from said second target surface by means of the impact of saidsecond pulsed beam of electrons and plasma against said second targetsurface; and

depositing simultaneously said metal oxide and said doping agent on saiddeposition surface by means of the contact of said plume of metal oxideand of said plume of doping agent with said deposition surface, therebya film of said metal oxide doped with said doping agent is obtained onsaid deposition body.

In one embodiment of this aspect of the invention, the metal oxide usedin this method is, for example, type-p ZnO and the doping agent is, forexample, a Li containing compound, as Li2O.

In another embodiment of this aspect of the invention, the metal oxideused is ZnO and the doping agent comprises magnetic species.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail with reference to thefigures that follow.

FIG. 1 a: diagram of the electron beam source of the pulsed plasmadeposition PPD device.(the current “trigger” is external). The electronbeam source will be also designated herein as PPD gun, electron gun orgun.

FIG. 1 b: ablation effect and plasma generation of PPD on a zinc oxidetarget (both the primary plasma of the electron pulse in the glasscapillary and the secondary plasma of target material created by meansof the microexplosion produced by the arrival of the packet of electronson the surface of the target can be seen).

FIG. 2: schematic description of the ablation process (the picture onthe left describes the situation of the arrival of the packet ofelectrons on the surface of the target and the image on the rightdescribes the situation of microexplosion of the surface, i.e., theablation of the material).

FIG. 3: ZnO film resistivity as a function of the oxygen pressure in thedeposition chamber and of the temperature of the substrate, in which theoptimum deposition parameters can be identified with respect to theminimum of resistivity ρ=0.6 mΩ-cm (p=6×10⁻⁵ mbar, T=300° C.).

FIG. 4: example of the measurement of transparency for wavelengths from400 to 800 nm of a ZnO film. The oscillations caused by the flat andparallel arrangement of the film surfaces can be observed; the graycurve is a polynomial fit in order to determine the average value oftransparency in each point of the wavelength range.

FIG. 5: example of the measurement of transparency for wavelengths inthe IR range of a ZnO film. The film was deposited on a ZnSe crystal forreasons of transparency of the substrate in the IR range.

FIG. 6: dependency of the transparency of ZnO film for the wavelengthλ=550 nm (visible range) on the deposition parameters. A strongdependency of transparency at this wavelength on deposition parametersis not observed. A weak minimum of transparency can be seen fortemperatures above 400° C.

FIG. 7: dependency of the transparency of ZnO film for the wavelengthλ=750 nm (infrared range) on the deposition parameters. Transparency inthe IR follows the electrical resistivity in the most evident form: theminimum of transparency corresponds approximately to the minimum ofresistivity.

FIG. 8: example of the morphology of a ZnO film deposited on a glasssubstrate at the temperature T=300° C. and at an oxygen pressurep=5·10⁻⁵ mbar. The film demonstrates the low resistivity of 1.11 mΩ-cmand a low roughness of 8 nm.

FIG. 9: example of the measurement of transparency for wavelengths from400 to 800 nm of an AZO film.

FIG. 10: scheme of the two-gun PPD system; 1—deposition chamber,2—target of the PPD gun 1, 3—target of the PPD gun 2, 4—sample(deposited thin film), 5—PPD gun 1 as disclosed in the FIG. 1, 6—PPD gun2 as described in the FIG. 1.

FIG. 11: picture of the working two-gun PPD system.

WAYS OF CARRYING OUT THE INVENTION

A schematic representation of an apparatus in which the method accordingto the first aspect of the present invention can be provided is shown inFIG. 1.

A schematic representation of an apparatus in which the method accordingto the second aspect of the present invention can be provided is shownin FIG. 10.

In one of its aspects, the present invention relates to the depositionof TCO by adapting a pulsed plasma deposition technique (PPD) based onthe generation of high-energy electron pulses (up to 20 keV) and ofplasma created by a working gas, such as oxygen, argon or nitrogen, atlow pressure (from 10⁻⁶ to 10⁻² mbar), disclosed in WO2006/105955 andincluded herein by reference, together with an apparatus suitable forgenerating such pulses. The diagram of the apparatus used is shown inFIG. 1.

The working principle of the PPD system (which is not a part of thepresent invention) is similar to that one presented in the patentDE2804393 (U.S. Pat. No. 4,335,465 (A1)). However, the preferredembodiment of the PPD system used for the present invention is totallydifferent from that one presented in the patents DE2804393 (U.S. Pat.No. 4,335,465 (A1)), U.S. Pat. No. 5,576,593A and the patentapplications US2005012441A1 and US20070026160A1.

Electrons and plasma generated near the hollow cathode are subtractedand are accelerated with the electrical potential difference (up to 20kV) between the hollow anode and the cathode and pass within thecapillary or tube made of glass in an equipotential region between theanode and the target. By means of the impact of the packet (beam) ofaccelerated electrons and plasma on the surface of a target, the energyof the packet (beam) is transferred into the material of the target andcauses its ablation, i.e., the explosion of the surface in the form of aplasma of the target material, also known as “plume”, which propagatesin the direction of a substrate or support on which it is deposited(FIG. 2).

The ion conductivity of the low-pressure gases ensures electrostaticshielding of the space charge generated by the electrons. As aconsequence of this, self-sustained beams can be accelerated with highenergy and power density and directed against a target which is kept atGROUND potential, thus causing explosions below the surface of thetarget which generate the expulsion of material from such target(ablation or “explosive sublimation” process), thus forming the plumewhich propagates normally to the surface of the target.

The ablation depth is determined by the energy density of the beam, bythe duration of the pulse, by the vaporization heat and by the heatconductivity of the material that constitutes the target as well as bythe density of such target.

The material of the plume, during its path between the surface of thetarget and of the substrate, interacts with the carrier gas provided inthe deposition chamber at low pressure (from 10⁻⁶ to 10⁻² mbar) and canbe either left unchanged or slightly oxidized (carrier gas—oxygen),unchanged or slightly reduced (carrier gas—argon, nitrogen) or doped (asin the case of ZnO and the carrier gas mixture of nitrogen and NO).Recently it has been demonstrated (Krasik Ya. E., Gleizer S., Chirko K.,Gleizer J. Z., Felsteiner J., Bernshtam V., Matacotta F. C., J. Appl.Phys., 99, 063303, (2006)) that only a small part (approximately 1%) ofthe electrons of the packet is accelerated by means of the fulldifferential of potential between the cathode and the anode. The energyof most of the electrons does not exceed 500 eV. The deposition rate ofthe material (film growth rate) can be controlled by means of thefrequency at which the electron packets are generated (repetitionfrequency), the difference of the potential between the cathode and theanode and the corresponding average current (approximately 3-5 mA) andby means of the distance between the target and the substrate.

The inventors of the present invention have found that in order tooptimize the growth of the film on the substrate it is possible, amongother things, to select and fix the suitable temperature of thesubstrate, for example by means of a heater incorporated in thesubstrate holder.

The inventors of the present invention have further found that by usingpulsed beams of electrons and plasma it is possible to deposit metaloxide films, particularly films and thin films of transparent conductingoxides, on a rigid or flexible surface made of inorganic or organicmaterial, which have a smooth surface, high optical transparency and lowelectrical resistivity and are suitable for use in devices such as OLEDsor solar cells.

Transparent conducting oxides have been deposited by means of the methodaccording to the present invention with a purchase cost of the apparatuswhich is significantly lower than a PLD system, a production cost (interms of cost of the electricity used) of no more than 10% of theproduction cost using PLD and with a system maintenance cost which isnegligible with respect to the PLD system, with a higher ablationprocess efficiency than the PLD method, and with good scalability. Theprocess according to the present invention in fact can be performedsimply and inexpensively. The use of more than one gun to provide asystem can be implemented easily. Moreover, this system does not exhibitthe problems of adaptation of the dimensions of the apparatus to thedimensions of the required deposition process.

In the methods according to the present invention, the beam of electronsand plasma preferably has a pulsed energy from 500 eV to 50 keV,particularly from 5 keV to 20 keV.

In the deposition chamber there is a working gas, which is preferablyselected from the group constituted by oxygen, argon, nitrogen andspecial mixtures such as methane in argon, hydrogen in argon, boranes,diboranes, ammonia, et cetera.

Preferably, a pressure from 10⁻⁶ to 10⁻² mbar, preferably from 10⁻⁵ to5x 10⁻³ mbar, is maintained in the deposition chamber.

The beam of electrons and plasma used in the methods according to thepresent invention is preferably a pulsed beam of electrons and plasmagenerated at a frequency from 0.1 to 500 Hz, particularly from 1 to 19Hz.

Preferably, the pulsed electron and plasma beam used in the methodsaccording to the present invention is generated by using an averagecurrent from 1 to 50 mA, particularly from 1 to 5 mA.

The pulsed beam of electrons and plasma is a beam of electrons andplasma generated by using a potential difference between an anode and acathode preferably from 500 V to 50 keV, particularly from 12 to 18 keV.

The methods according to the present invention can further comprise astep for adjusting a distance between said target surface and saiddeposition surface.

Preferably, the target surface and the deposition surface are arrangedat a mutual distance of 5 to 500 mm.

The methods according to the present invention can further comprise thestep for adjusting the temperature of said supporting body.

The temperature of the supporting body is preferably fixed in a rangefrom ambient temperature to 550° C., more preferably at a temperaturefrom ambient temperature to 350° C.

Moreover, the target body and the supporting body are positioned in thedeposition chamber so that the deposition surface lies on thepropagation path of the plume of metal oxide ablated from the targetsurface, which makes contact with the deposition surface so as to formby deposition the metal oxide film on the deposition surface.

The target body and/or the supporting body can be subjected to rotarymotion during such deposition step in order to achieve more uniformdeposition.

The thickness of the film deposited with the methods according to thepresent invention can be preset and controlled by means of aquartz-crystal balance. Preferably, the film deposited with the methodaccording to the present invention is a thin film, preferably with athickness in the range from 1 to 500 nm. More preferably, the thicknessof the film deposited with the method according to the present inventionis on the nanometer scale, particularly 200 nm.

All the considerations above are valid both for the method of thepresent invention using one PPD gun and the method of the presentinvention using two or more PPD guns and allowing simultaneousdeposition on the deposition support of more substances.

The following exemplary of embodiments of the present invention areprovided by way of non-limiting examples of the present invention.

Deposition of ZnO Optimized Parameters of ZnO Deposition, Properties ofDeposited Films

Experiments of deposition of ZnO films with the method according to thepresent invention have been performed with deposition parametersselected in the pressure range 1×10⁻⁵5×10⁻³ mbar of oxygen in thedeposition chamber and a substrate temperature from 100 to 500° C.Optical microscope slides, quartz windows, ZnSe crystals and flexiblesheets (PC, PTFE, PET) were used as substrates. The electron gunparameters were kept within the voltage ranges 12-18 kV, the powersupply current within 3-5 mA, and the frequency of the electrondischarges within 1-10 Hz. During deposition, the target was turned inorder to prevent possible alteration of the chemical composition of thesurface. The substrate was kept motionless during deposition and heatedby using the halogen lamp. The temperature was measured by means of thethermocouple attached to the holder of the substrate, close to thesubstrate (between the substrate and the holder). The average depositiontime was selected as 2 hours (the growth rate of the film thickness ison average 0.2 A/s).

The physical properties of the ZnO films were studied by measuringelectrical resistivity (van de Pauw method), optical transparency (bymeans of the JASCO 550V spectrometer and the Bruker IFS-88Fourier-transform interferometer) in the visible and infrared wavelengthrange, Hall effect, scanning electron microscopy and AFM (atomic forcemicroscope).

The films deposited in the conditions specified above exhibit athickness from 20 to 200 nm, an electrical resistivity from 1 mΩcm to 95mΩcm, a transparency from 78 to 97%, a crystalline film morphology and arelatively low roughness, from 8 to 10 nm.

In particular, the following results were achieved for films of ZnOdeposited by using the following conditions:

pulsed electron and plasma acceleration voltage V=−16 kV

deposition time t=2 hours

deposition frequency f=2 Hz

distance between target and substrate d=40 mm.

Electrical Resistivity

Hall measurements have demonstrated that ZnO films are n-typesemiconductors with a concentration of free charge carriers on the orderof 10²⁰−10²¹ cm⁻³.

FIG. 3 summarizes the measurements of electrical resistivity for filmsdeposited on a rigid support (glass) for different oxygen pressures anddifferent substrate temperatures. As can be seen, the surface of thethree-dimensional chart which corresponds to the values of resistivityfor different combinations of parameters of the deposition demonstratesthe minimum (value of resistivity ρ=0.6 mΩ-cm) neighborhood of thepressure values 6×10⁻⁵ mbar of oxygen and the temperature of thesubstrate 300° C.

Films deposited in the same conditions but at ambient temperature on aflexible substrate (PC) demonstrate the minimum resistivity value ρ=2.4mΩ-cm.

Transmittance

The examples of the measurements of transparency in UV-Vis are shown inFIGS. 4, 5, 6 and 7. The average value of the transparency of the ZnOfilms deposited on a rigid support (glass) at a pressure of 1*10⁻⁴ mbarand at a substrate temperature of 500° C. is T=93% in the 400-800 nmwavelength range (FIG. 4). In the wavelength range from 2.5 to 10 μm,the transparency of the film deposited on ZnSe crystal is from 85 to 47%(FIG. 5).

By varying the deposition parameters as in the resistivity example, oneobtains that the transparency of the films deposited on the glasssubstrate varies from 78% to 97% at the wavelength of 550 nm (FIG. 6)and from 87 to 97% at the wavelength of 750 nm (FIG. 7).

Morphology

FIG. 8 shows the example of the morphology of films deposited on glassor quartz substrates and studied by means of the scanning electronmicroscopy method. FIG. 8 shows a film deposited at a glass substratetemperature T=300° C. and an oxygen pressure in the deposition chamberof p=5*10⁻⁵ mbar. The morphology of the film corresponds to that of acrystalline film with low surface roughness (typical ZnO film depositedalso by means of a method such as PLD) with a thickness of 200 nm. Thefilm demonstrates low resistivity due to high crystallinity of the film(relaxation of structural disorder) ρ=1.11 mΩ-cm.

Roughness

The morphological measurements of the ZnO deposited films obtained bymeans of the AFM method have revealed the relatively low roughness (8-10nm on a thickness of 180-200 nm) and the presence of a small number ofdefects such as pinholes of the films.

The ZnO film with the lowest resistivity was deposited by using thefollowing conditions:

oxygen pressure in the deposition chamber p=5*10⁻⁵ mbar

substrate temperature T=300° C.

pulsed electron and plasma acceleration voltage V=−16 kV

deposition time t=2 hours

deposition frequency f=2 Hz

distance between target and substrate d=40 mm

The following results were achieved:

film thickness s=200 nm

resistivity ρ=1.11 mΩ-cm

Deposition of AZO Optimized Parameters of AZO Deposition, Properties ofDeposited Films

The deposition parameters of AZO films (98% ZnO and 2% Al by weight)were selected equal to those indicated above for ZnO.

The physical properties of AZO films were studied by means of themeasurements of electrical resistivity (van de Pauw method), opticaltransparency in the visible and infrared wavelength range.

Electrical Resistivity

Films of zinc oxide doped with aluminum (AZO) deposited on glassdemonstrate a similar dependency on deposition parameters (substratetemperature, oxygen pressure in the deposition chamber) as ZnO films.The minimum of resistivity (ρ=0.16 mΩ-cm) is achieved for pressureparameters p=2×10⁻⁵ mbar of oxygen and the substrate temperature T=300°C., the pressure of the resulting film being 50 nm.

Transmittance

For AZO films deposited on a rigid support (glass) at a pressure of2.5·10⁻⁵ mbar and at a substrate temperature of 300° C., the averagevalue of transparency is T=91% in the 400-800 nm wavelength range (FIG.9).

The AZO film with the lowest resistivity was deposited by using thefollowing conditions:

oxygen pressure in the deposition chamber p=2·10⁻⁵ mbar

substrate temperature T=300° C.

pulsed electron and plasma acceleration voltage V=−16 kV

deposition time t=2 hours

deposition frequency f=2 Hz

distance between target and substrate d=40 mm

The following results were achieved:

film thickness s=50 nm

resistivity ρ=0.167 mΩ-cm

Deposition of Doped Zinc Oxide by Multiple Ablation. PPD Ablation withMultiple Guns

Deposition of doped material or a material grown by kinematic means (asystem not in thermodynamic equilibrium) requires the use of two or moreguns working simultaneously. One of the guns is used to deposit the basematerial and the others are then used for ablation and deposition of thedoping materials in the suitable quantities. Such system allows tocreate alloys and dopings of systems which cannot be created in bulkform (for example due to phase separation, which prevents thiscombination of the materials or the selected concentrations of dopants).Moreover, it is possible to create systems grown in conditions of lackof thermodynamic equilibrium (such as for example amorphous systems orcrystalline systems but with structurally incompatible dopantsincorporated kinematically—for example zinc oxide doped with themagnetic species—Fe, Mn, Co, Ni and the like).

The PPD system of two or more guns is composed, in addition to the partsalready mentioned for the single-gun system, of two or more guns withthe corresponding power supplies and the unit for mutual synchronizationand “timing” of the guns. The synchronization and “timing” unit performstwo functions. The first function must ensure the required ratio betweenthe amount of base material and dopants by controlling the frequenciesof the deposition of the corresponding guns. The second function relatesto the “timing” of the formation of the plume of base material anddopant. The sequence of the events for ablation of the base material andof the dopant must be such as to ensure the overlap of the plumes of thetwo materials which is suitable to provide the sought chemical reactionsin the plasma phase. The interval between the ablation of one materialand ablation of the other material varies between 0 and 500 ns,depending on the combination of materials and on the type of reactionexpected.

Deposition of P-Type ZnO Doped with Li by Using Two Guns

The ZnO material mentioned above are all of the n-type (i.e theelectrons are the majority charge carriers). In the subsequent sectionthe p-type of ZnO is used. In the p-type ZnO, the holes are the majoritycharge carriers.

The PPD system with two PPD guns is demonstrated in FIG. 10. Each gunhas its own target: one is pure ZnO and the second one can be composedof ZnO and the dopant (as lithium oxide) at different concentrations orpure lithium oxide (the composition of the second target is(Li₂O)_(x)+(ZnO)_(1-x), where 0≦x≦1; preferably, 0.03≦x≦0.1). The amountof dopant (lithium) is controlled by means of the concentration of thedopant in the target and by means of the ratio between the ablationfrequency of the base material (ZnO) and the dopant((Li₂O)_(x)+(ZnO)_(1-x)). The plumes generated by two correspondingtargets overlap on a substrate which is fixed on, and heated by, aheating substrate carrier. The temperature of the heating unit can becontrolled from ambient temperature to 500° C., preferably from 150° C.to 350° C. During ablation and deposition, the target and the substraterotate at a frequency from 0.1 to 5 Hz, preferably at a frequency from0.5 to 1 Hz. The deposited material forms a thin film of ZnO doped withLi with a thickness from 10 to 1000 nm, preferably from 100 to 200 nm.The remaining deposition parameters are equal to the ones for thedeposition of n-type ZnO films mentioned above.

Hall-effect measurements demonstrate that the conductivity of films ofZnO doped with Li prepared by means of the PPD method is of the p typewith a concentration of charge carriers (holes) of approximately 5*10¹⁷cm⁻³, mobility is approximately 1.7 cm²/Vs and typical resistivity is6.2 Ωcm.

1. A method for depositing a metal oxide film on a surface of a supporting body for said film, comprising the steps of: providing a deposition chamber; providing a pulsed beam of electrons and plasma in said deposition chamber; supplying a supporting body in said deposition chamber, said supporting body having a deposition surface; providing a target body made of a material which comprises said metal oxide in said deposition chamber, said target body having a target surface; providing a plume of metal oxide ablated from said target surface by means of the impact of said pulsed beam of electrons and plasma against said target surface; and depositing a metal oxide film on said deposition surface by means of the contact of said plume with said deposition surface.
 2. The method according to claim 1, wherein said metal oxide is a transparent conducting oxide, particularly a metal oxide selected from the group constituted by zinc oxide and zinc oxide doped with aluminum, or lithium or other dopants.
 3. The method according to claim 1, wherein said supporting body is a supporting body made of a transparent material, or a non transparent material.
 4. The method according to claim 1, wherein said supporting body is a supporting body which is flexible or rigid.
 5. The method according to claim 1, wherein said supporting body is a body made of solid inorganic material.
 6. The method according to claim 5, wherein said supporting body is made of a material selected from the group constituted by glass, quartz, CdS, ZnSe, metals, and inorganic semiconductors.
 7. The method according to claim 1, wherein said supporting body is a body made of solid organic material.
 8. The method according to claim 7, wherein said supporting body is made of a material selected from the group constituted by polymers such as polyesters, polyolefines, polyimides, phenolic resins, polyanhydrides, conducting polymers, conjugated polymers, fluoropolymers, silicone rubbers, silicone polymers, biopolymers, copolymers, block copolymers such as polycarbonate, PTFE, PET, PNT, PEDOT, polyaniline, polypyrrole, polythiophenes, polyparaphenylenes (PPV), polyfluorenes, and molecular solids like molecular semiconductors, molecular crystals, molecular thin films, molecular dyes, such as AlQ3, thiophene oligomers, PPV oligomers, pentacene, tetracene, rubrene, NPB, fullerenes, carbon nanotubes and fullerides.
 9. The method according to claim 1, wherein said electron and plasma beam has a pulsed energy from 500 keV to 50 keV, particularly from 5 keV to 20 keV.
 10. The method according to claim 1, wherein a pressure from 10⁻⁶ to 10⁻² mbar, preferably from 10⁻⁵ to 5×10⁻³ mbar, is maintained in said deposition chamber.
 11. The method according to claim 1, wherein in said deposition chamber there is a working gas selected among the group constituted by oxygen, argon, nitrogen and mixtures of methane and argon, hydrogen and argon, boranes, diboranes and ammonia.
 12. The method according to claim 1, wherein said beam of electrons and plasma is a pulsed beam of electrons and plasma generated with a frequency from 0.1 Hz to 500 Hz, particularly from 1 Hz to 19 Hz.
 13. The method according to claim 1, wherein said pulsed beam of electrons and plasma is a beam of electrons and plasma generated by using an average current from 1 mA and 50 mA, particularly from 1 to 5 mA.
 14. The method according to claim 1, wherein said pulsed beam of electrons and plasma is a beam of electrons and plasma generated by using a potential difference between an anode and a cathode from 500 V to 50 ke V, particularly from 12 to 18 kV.
 15. The method according to claim 1, wherein said target surface and said deposition surface are arranged at a mutual distance from 5 mm to 500 mm.
 16. The method according to claim 5, wherein said support has a temperature comprised between ambient temperature and 550° C.
 17. The method according to claim 7, wherein said support has a temperature comprised between ambient temperature and 350° C.
 18. The method according to claim 1, wherein it further comprises the step for adjusting a distance between said target surface and said deposition surface.
 19. The method according to claim 1, wherein it further comprises the step for adjusting the temperature of said supporting body.
 20. The method according to claim 1, wherein said target body is subjected to a rotary motion during said deposition step.
 21. The method according to claim 1, wherein said deposition supporting body is subject of rotary motion during said deposition step.
 22. The method according to claim 1, wherein said supporting body and said target body are positioned within said deposition chamber so that said plume makes contact with said deposition surface.
 23. A method for depositing a film of a metal oxide doped with a doping agent on a surface of a supporting body for said film, comprising the steps of: providing a deposition chamber; providing a first and a second pulsed beam of electrons and plasma in said deposition chamber; supplying a supporting body in said deposition chamber, said supporting body having a deposition surface; providing in said deposition chamber a first and a second target body, said first target body being made of a material which comprises said metal oxide, said second target body being made of a material which comprises said doping agent, said first target body having a first target surface and said second target body having a second target surface; providing a plume of metal oxide ablated from said first target surface by means of the impact of said first pulsed beam of electrons and plasma against said first target surface, and a plume of said doping agent ablated from said second target surface by means of the impact of said second pulsed beam of electrons and plasma against said second target surface; and depositing simultaneously said metal oxide and said doping agent on said deposition surface by means of the contact of said plume of metal oxide and of said plume of doping agent with said deposition surface, thereby a film of said metal oxide doped with said doping agent is obtained on said deposition body.
 24. The method according to claim 23, wherein said metal oxide is type-p ZnO and said doping agent is a Li containing compound, as Li2O.
 25. The method according to claim 23, wherein said metal oxide is ZnO and said doping agent comprises magnetic species.
 26. A metal oxide film which can be obtained by deposition on a surface of a supporting body by means of the method according to claim
 1. 